Synthesis and redox properties of novel ferrocenes with redox active 2,6-di-tert-butylphenol fragments: The first example of 2,6-di-tert-butylphenoxyl radicals in ferrocene system

Article abstract of DOI:10.1016/j.jorganchem.2007.08.013

Novel Schiff bases of ferrocenecarboxaldehyde bearing 2,6-di-tert-butyphenol fragments N-(3,5-di-tert-butyl-4-hydroxyphenyl)iminomethylferrocene (1) and N-(3,5-di-tert-butyl-4-hydroxybenzyl)iminomethylferrocene (2) have been synthesized and characterized.

Full text of DOI:10.1016/j.jorganchem.2007.08.013

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Journal of Organometallic Chemistry 692 (2007) 5339–5344
www.elsevier.com/locate/jorganchem
Synthesis and redox properties of novel ferrocenes with redox active
2,6-di-tert-butylphenol fragments: The first example of
2,6-di-tert-butylphenoxyl radicals in ferrocene system
N.N. Meleshonkova, D.B. Shpakovsky, A.V. Fionov, A.V. Dolganov,
*
T.V. Magdesieva, E.R. Milaeva
Department of Organic Chemistry, Lomonosov Moscow State University, Moscow 119992, Russia
Received 25 December 2006; received in revised form 11 July 2007; accepted 10 August 2007
Available online 15 August 2007
Abstract
Novel Schiff bases of ferrocenecarboxaldehyde bearing 2,6-di-tert-butyphenol fragments N-(3,5-di-tert-butyl-4-hydroxyphenyl)imi-
nomethylferrocene (1) and N-(3,5-di-tert-butyl-4-hydroxybenzyl)iminomethylferrocene (2) have been synthesized and characterized.
The oxidation of the compounds 1 and 2 by PbO2 in solution leads to the formation of stable phenoxyl radicals 10 and 20 studied by
EPR spectroscopy. The redox properties of ferrocenes 1 and 2 were studied using cyclic voltammetry.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Ferrocene; Phenoxyl radicals; Redox reactions; EPR; Cyclic voltammetry
1. Introduction
can be achieved by using a redox active organic ligand
[8].
A family of ferrocene compounds are used as catalysts
[1,2], sensors [3], luminescent systems, and molecular elec-
tronic devices [4] due to their well established redox prop-
erties. The application of ferrocene derivatives in biological
areas and medicine is intensively investigated [5]. It is sup-
posed that ferrocenes participate in biochemical redox pro-
cesses, promoting formation of reactive oxygen species and
other free radical metabolites [6].
Previously, we have demonstrated that the introduction
of 2,6-di-tert-butylphenol into transition metal complexes
leads to the formation of stable polytopic redox systems
which can be oxidized either on metal or ligand centers
[9,10]. The involvement of the phenolic group or metal
ion in oxidation of these complexes is manifested in the for-
mation of polyfunctional catalytic systems which show
positive and/or negative catalytic effects [11].
Most of ferrocene derivatives undergo reversible one-elec-
tron oxidation forming ferrocenium cations (radical-cations)
due to the redox activity of metal center. Construction of
polytopic redox systems containing more then one redox
center has attracted considerable interest, both from the
fundamental and practical points of view [7]. The regula-
tion of the electronic communication between two centers
In this paper, we present the first example of Schiff bases
of ferrocenecarboxaldehyde with conjugated 1 and non-
conjugated 2 2,6-di-tert-butylphenol groups.
2. Results and discussion
Schiff bases 1, 2 were synthesized by the interaction of
ferrocenecarboxaldehyde with the corresponding amines
(Scheme 1). Phenyl and benzyl derivatives 3, 4 were chosen
as model compounds for the comparative electrochemical
study, in order to establish the role of phenol pendants in
*
Corresponding author. Tel.: +7 495 9393864; fax: +7 495 9395546.
E-mail address: milaeva@org.chem.msu.ru (E.R. Milaeva).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.08.013

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N.N. Meleshonkova et al. / Journal of Organometallic Chemistry 692 (2007) 5339–5344
Scheme 1.
oxidation processes and they were prepared using the
methods described earlier [12,13].
To prevent the oxidation of phenol containing amine,
the synthesis of compound 1 was carried out in ethanol
at room temperature using 4-amino-2,6-di-tert-bytylphenol
generated in situ from a more stable corresponding hydro-
chloride in the presence of NEt3. The compounds 1–4 were
isolated as orange solids, stable in the air and in solutions.
They were characterized by elemental analysis, IR, UV–vis,
1H and 13C NMR spectroscopies. The IR spectra of com-
plexes 1, 2 exhibit absorption bands at 3639 and
3627 cmꢀ1 (mO–H), and at 1654 and 1643 cmꢀ1 (mC@N) cor-
respondingly characteristic for non-associated sterically
hindered phenols. Electronic absorption spectra show the
intramolecular charge transfer bands in the ligand-metal
system at ꢁ450 nm. As it follows from the spectral data,
no oxidation of phenol substituents to the quinoid deriva-
tives occurs in the synthesis of ferrocenes 1, 2.
The chemical oxidation of phenol derivatives 1, 2 was
carried out in toluene using lead dioxide yielding phenoxyl
radicals 10, 20 (Scheme 2).
Fig. 1. Experimental 1 and simulated 2 EPR spectra of radical 10 (toluene,
293 K).
The X-band EPR spectra measured at 293 K show the
spin density distribution in the organic ligands. The exper-
imental and simulated EPR spectra (Fig. 1) of radical 10
exhibit 9 lines corresponding to the coupling of the
unpaired electron with two equivalent meta-protons of
the phenoxyl ring (1H) and nitrogen atom of azomethine
group (14N). The isotropic g-value for the radical 10 is
2.0055 and hyperfine coupling constants (ai) are:
a(2H) = 1.0 G; a(14N) = 5.9 G. The radical 10 is stable at
room temperature in the absence of dioxygen for several
days. The EPR spectrum of radical 20 shows the multiple
signal (g = 2.0030) with hyperfine coupling constants:
a(H) = 11.0 G (–CH2– group), a(2H) = 1.7 G (meta-pro-
tons of the phenoxyl ring), a(14N) = 1.5 G (Fig. 2).
Previously a number of analogues organic species – phe-
noxyl radicals generated in the oxidation of the corre-
sponding Schiff bases of substituted benzaldehydes has
been studied [14,15]. The stability and the values of hyper-
fine splitting constants of these radicals are influenced by
the electron-withdrawing or electron-donating character
of the para-substituent in the phenyl ring. For instance,
the values of a(14N) are 4.8 and 3.4 G for para-methoxy-
and para-nitrophenyl derivatives correspondingly. The fer-
rocene unit with Fe(II) center is electron-donating group
[1] and increases the stability of radicals 10 and 20 as it
gas been shown by EPR. The analogues effect has been
observed previously in P-centered radical-cations [16] and
C-centered triphenylmethyl radicals [17] possessing ferroce-
nyl groups.
The high stability of species 10 and 20 is in agreement
with a certain degree of electronic delocalization over the
molecule.
The oxidation of ferrocene derivatives 1 and 2 by
AgSO2CF3 in chloroform gives purple solutions, and the
deposition of Ag is observed. However, we failed to isolate
the corresponding ferrocenium salts. This reaction was
1
monitored by H NMR in CDCl3 at room temperature.
Scheme 2.
The disappearance of the OH group’s protons signals in

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5341
Redox properties of the phenol derivatives 1, 2 as well as
of their phenyl analogues 3, 4 were studied by cyclic vol-
tammetry in CH3CN. The oxidation potentials are given
in Table 1. The compounds 3, 4 containing phenyl groups
display typical for ferrocenes one-electron reversible oxida-
tion at almost identical potential values which is attributed
to Fe2+/Fe3+ transformation (Fig. 3).
Ferrocenes 1 and 2 show more complex electrochemical
behavior due to the presence of two redox active fragments –
phenol and ferrocene moieties. In the case of 1 these frag-
ments are conjugated and linked via –N@CH– bridge. In
the case of 2 the linker also contains –CH2– fragment
which may impede redistribution of electron and spin den-
sity between the two fragments. Compounds 1 and 2 show
three oxidation peaks: one-step irreversible and two-steps
reversible redox waves.
In the case of 1 the first irreversible one-electron step can
be attributed to ferrocene-centered oxidation. The oxida-
tion potential (E1 = 0.48 V) is comparable with that of
unsubstituted ferrocene ðEþFc=Fc ¼ 0:51 V, [18]).
As it has been shown previously for a series of para-
substituted 2,6-di-tert-butylphenols, the first irreversible
oxidation potentials are observed at much higher values
[19]. For 2,6-di-tert-butyl-4-methylphenol and 2,6-di-tert-
butyl-4-carboxyphenol, for instance, these values are 1.21
and 1.68 V, respectively, versus. SCE [18].
The absence of the corresponding reduction peak in the
reverse scan means that primarily formed ferrocenium cat-
ion disappears due to some chemical transformations
(Scheme 3). Since 2,6-di-tert-butylphenol is the electron-
donating group, the most probable pathway is intramolec-
ular reduction of iron(III) center. The electrochemically
induced intramolecular electron transfer (IET) results in
the formation of the radical-cation at the phenol group,
followed by proton abstraction. The analogues IET process
was observed previously for p-cyclopentadienyl iron com-
plexes associated with the intramolecular rearrangement
of the ligand to metal system [20]. The next reversible
one-electron step corresponds to the oxidation of ferrocene
moiety at higher potential (E2 = 0.75 V).
The third oxidation step is associated with a well-known
reversible process of one-electron oxidation of phenoxyl
radical [21,22] and is observed at ꢁ1 V due to the presence
of electron-withdrawing ferrocenium substituent. Since X-
band EPR spectra of ferrocenium species are observed at
cryogenic temperature due to rapid relaxation [16,23] we
failed to register the corresponding products formed by
in situ EPR spectroscopy.
Fig. 2. Experimental 1 and simulated 2 EPR spectra of radical 20 (toluene,
293 K).
NMR spectra confirms the assumption that the chemical
oxidation is attributed to the phenol moiety that leads to
the formation of quinoid derivatives as decomposition
products.
As it was shown previously using cyclic voltammetry
[18], the addition of antioxidants such as 2,6-di-tert-
butyl-4-methylphenol or 3,5-di-tert-butyl-4-hydroxyani-
sole to ferrocene solutions saturated with O2 in basic
media prevents the complete decomposition of electrogen-
erated ferrocenium cation. It was proposed that the role
of phenols is the reduction of initially formed ferroce-
nium cation.
Redox behavior of compound 2 is analogues. Cyclic vol-
tammogram of 2 (Fig. 3c) shows three redox processes at
oxidation potentials slightly shifted (DE1 = 0.16, DE2 =
0.06, DE3 = 0.04 V, respectively). Second and third revers-
ible oxidation peaks are even more pronounced. Therefore,
the presence of –CH2– linking group does not prevent the
ability of 2,6-di-tert-butylphenol to act as an intramolecu-
lar electron donor. To confirm the assumption that phenol
substituent plays a role of the intramolecular one-electron
Table 1
Electrochemical data for compounds 1–4a
Compound
E1 ꢂ ðVÞ
E2 ꢂ ðVÞ
0.75/0.67
0.81/0.75
–
–
E3 ꢂ ðVÞ
1.01/0.92
1.05/0.97
–
–
1
2
3
4
a
0.48 (irr.)
0.64 (irr.)
0.72/0.64
0.66/0.59
Pt, CH3CN, concentration of compounds 1–4 are 0.7 mM, 50 mM
Bu4NBF4, vs. Ag/AgCl/KCl, scan rate = 0.1 V sꢀ1, 293 K.

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N.N. Meleshonkova et al. / Journal of Organometallic Chemistry 692 (2007) 5339–5344
Fig. 3. Cyclic voltammograms of 0.7 mM of 1 (a), 2 (b), 3 (c), and 4 (d) at 293 K. Conditions: Pt, CH3CN, 50 mM Bu4NBF4, vs. Ag/AgCl/KCl, scan
rate = 0.1 V sꢀ1
.
Scheme 3.
donor an additional experiment was carried out. 2,4,6-Tri-
tert-butylphenol was added to the solution of 3 in CH3CN
and cyclic voltammogram was recorded. However, no
reduction of ferrocenium cation by phenol was observed.
Thus, it can be proposed that the role of phenol group in
ferrocene redox behavior is associated with the intramolec-
ular effect of one-electron donor.
3. Conclusion
The novel ferrocene derivatives with 3,5-di-tert-butyl-4-
hydroxyphenyl substituents exhibit the properties of multi-
step redox systems and may serve as models for the study of
intramolecular electron transfer between two redox active
sites of the molecule (the phenol and ferrocene groups).

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5343
4. Experimental
1H NMR (400 MHz, CDCl3): d 8.26 (s, CH@N, 1H);
7.13 (s, ArH, 2H); 5.15 (s, ArOH, 1H); 4.70 (s, Hfer, 2H);
4.58 (s, ArCH2–, 2H), 4.38 (s, Hfer, 2H); 4.18 (s, C5H5,
5H); 1.47 (s, C(CH3)3, 18H). 13C NMR (101 MHz, CDCl3):
d 161.30 (s, CH@N); 152.90 (s, Car–OH); 135.95 (s, C–
C(CH3)3); 128.35 (s, Car–CH2); 124.88 (s, C6H2); 73.19 (s,
C5H4); 70.38 (s, CH2); 69.97 (s, C5H4); 69.06 (s, C5H5);
68.61 (s, C5H4); 34.38 (s, C(CH3)3); 30.34 (s, C(CH3)3).
Elemental analysis: Anal. Calc. for C26H33FeNO: C,
72.39; H, 7.71; N, 3.25; Fe, 12.94. Found: C, 71.56; H,
7.77; N, 3.19; Fe 12.83%.
CDCl3 (Merck) was used without further purification,
other solvents were routinely distilled and dried prior to
use. Aniline, benzylamine and ferrocenecarboxaldehyde
were obtained commercially (Aldrich), 4-amino-2,6-di-tert-
butylphenol hydrochloride was prepared according to the
known procedure [24]. The H and 13C{H} NMR spectra
1
were recorded with a Bruker Avance-400 FT-spectrometer.
IR spectra were recorded with a IR200 (Thermo Nicolet)
spectrophotometer in KBr pellets. UV–vis spectra were
obtained with a Cary 219 Varian spectrophotometer in
CHCl3. EPR spectra were recorded with a Bruker EMX,
Bruker ELEXSYS E-500-10/12 spectrometers in the O-
band range (9.8 GHz). The measurements were carried out
after pre-evacuation of samples solutions (concentration
0.1 mM). The oxidant PbO2 (Aldrich) was taken in a tenfold
excess. Cyclic voltammetry experiments were performed in
CH3CN solution with 0.05 M Bu4NBF4 as supporting elec-
trolyte using a model IPC-Win potentiostat. A platinum
working electrode, platinum wire auxiliary electrode and
aqueous silver/silver chloride reference electrode were used.
IR (KBr pellet, cmꢀ1): m(OH) 3627, m(CH) 2870–2998;
m(C@N) 1643.
UV–vis, kmax, nm (lne): 448 (6.3); 336 (7.3).
4.3. N-phenyl-iminomethylferrocene (3)
Ferrocenecarboxaldehyde (0.28 g, 1.3 mmol) and aniline
(0.30 g, 1.3 mmol) were dissolved in ethanol (3 ml) and the
solution was left at 0 °C for 2 days. The precipitate formed
was filtered off, washed with hexane and dried in vacuo to
dryness. Yield of 3 0.25 g (67%), m.p.: 76–77 °C. (m.p.:
77 °C [12]).
4.1. N-(3,5-di-tert-butyl-4-hydroxyphenyl)-
iminomethylferrocene (1)
4.4. N-benzyl-iminomethylferrocene (4)
Ferrocenecarboxaldehyde (0.43 g, 2.01 mmol) and 4-
amino-2,6-di-tert-butylphenol hydrochloride (0.52 g, 2.02
mmol) were dissolved in ethanol (7 ml) and a few drops of
NEt3 were added. The mixture was stirred for 15 min at
room temperature, and then the orange precipitate was fil-
tered off, washed with hexane, and dried in vacuo. Yield of
1 0.56 g (66%), m.p.: 184–185 °C.
Ferrocenecarboxaldehyde (0.22 g, 1.0 mmol) and ben-
zylamine (0.21 g, 2.0 mmol) were dissolved in benzene
(10 ml). The reaction mixture was refluxed for 2 h, and left
at room temperature overnight. The reaction mixture was
rotary evaporated to dryness. The solid residue was washed
with hexane and dried in vacuo. Yield of 4 0.24 g (77%),
m.p.: 105–106 °C. (m.p.: 102 °C [13]).
1H NMR (400 MHz, CDCl3): d 8.35 (s, CH@N, 1H);
7.05 (s, ArH, 2H); 5.13 (s, ArOH, 1H); 4.81 (s, Hfer, 2H);
4.47 (s, Hfer, 2H); 4.26 (s, C5H5, 5H); 1.50 (s, C(CH3)3,
18H). 13C NMR (101 MHz, CDCl3): d 158.69 (s, CH@N);
136.66 (s, C–C(CH3)3); 117.39 (s, C6H2); 70.94 (s, C5H4);
69.64 (s, C5H4); 69.17 (s, C5H5); 68.81 (s, C5H4); 34.47 (s,
C(CH3)3); 30.29 (s, C(CH3)3).
Acknowledgements
This work was supported by the Russian Foundation
for Basic Research (Grant No. 05-03-32869), Russian Fed-
eral Agency on Science and Innovations (Contract
02.451.11.7012), Russian Federation President’s Grants
(RI-112/001/056).
Elemental analysis: Anal. Calc. for C25H31FeNO: C,
71.95; H, 7.49; N, 3.36; Fe 13.42. Found: 71.88; H, 7.49;
N, 3.65; Fe 13.28%.
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